Scanning electron beam computed tomography systems are described generally in U.S. Pat. No. 4,352,021 to Boyd, et al. (1982). The theory and implementation of devices to help control the electron beam in such systems is described in detail in U.S. patents to Rand et al. U.S. Pat. Nos. 4,521,900 (1985), 4,521,901 (1985), 4,625,150, (1986), 4,644,168 (1987), 5,193,105 (1993), and 5,289,519 (1994). Applicants refer to and incorporate herein by reference each said patent to Boyd et al. and to Rand et al.
As shown in FIGS. 1 and 2 and as described in detail in U.S. Pat. No. 4,521,900 to Rand et al., a generalized computed tomography X-ray transmission scanning system 8 includes a vacuum housing chamber 10 wherein an electron beam 12 is generated. The electron beam is caused to scan at least one circular target 14 located within front lower portion of chamber 16.
Upon striking the target, the electron beam emits a moving fan-like beam of X-rays 18. The X-rays pass at least partially through a region of a subject 20 (e.g., a patient or other object) and register upon a region of a detector array 22 located diametrically opposite. The detector array outputs data to a computer processing system (indicated by arrows 24) that processes and records the data. The computer processing system then reconstructs or produces an image of a slice of the subject on a video monitor 26. As indicated by the second arrow 24, the computer processing system also controls the scanning system and its production of the electron beam. By repeating the scanning process after the patient has been moved laterally along chamber Z-axis 28, a series of X-ray images representing axial "slices" of the patient's body is produced.
Referring to FIG. 2, more specifically, electron gun 32 is disposed within extreme upstream end 34 of chamber (or "drift tube") 10, which chamber may be as long as about 3.8 m in some prior art configurations. A sufficiently long drift tube permits the electron beam to expand and become more uniform, and can promote electron beam space-charge homogenization by evening out electron distribution.
In response to high voltage excitation (e.g., 130 kV) the electron gun produces electron beam 12. The high voltage electron gun potential accelerates the electron beam downstream along a first straight line path defining the chamber Z-axis. A beam optical system 38 typically includes a focus coil 40 and dipole and quadrupole coils 42, and is mounted downstream on chamber 10. Coils 40 and 42 respectively magnetically focus and shape and scan the beam 12 typically about 210.degree. in a scan path across the arc-like ring target 14.
Although vacuum pump 36 evacuates chamber 10, residual gases inevitably remain that produce positive ions in the presence of the electron beam. Gases may also be introduced into the chamber for the purpose of producing positive ions, since the ions are beneficial in the downstream chamber region.
The electrons are negatively charged and the resultant space-charge causes the electron beam to diverge or expand in the upstream chamber region between the electron gun and the focus and deflection coils. This upstream region expansion is beneficial because the beam width at the target varies approximately inversely with the beam diameter at the focus and deflection coils. However the positive ions that are created can detrimentally neutralize the space-charge, preventing electron beam divergence in the chamber upstream region. Unless counteracted, this can increase beam width at the target, resulting in a defocused X-ray image. Neutralization can also result in the electron beam becoming unstable and even collapsing completely.
In the chamber region downstream from the focus and deflection coils, a converging electron beam is desired. Here the beam preferably is neutralized by positive ions produced by the electrons from residual gas in the chamber, or from a gas purposely introduced into the chamber. The neutralization eliminates electron self-repulsion, while the beam's attractive magnetic field converges and self-focuses the beam. As a result, the beam can self-focus sharply upon the target to produce a sharp X-ray image. Elements of the beam optical system fine tune the converged beam to produce a sharp X-ray image.
Ideally, the electron beam would exhibit uniform current density, diverging upstream and converging to sharply self-focus downstream. An electron beam with a uniform electron distribution can act as its own perfect lens: self-diverging in the upstream chamber region and self-converging in the downstream chamber region to focus sharply on the target. A uniform space-charge density is desired because any optical aberrations due to the electron beam self-forces would then be eliminated. In addition to degradation from ions, the electron beam space-charge density may not be perfectly uniform due to imperfections in the electron gun and in the beam optics system.
Understandably, achieving a perfectly uniform space-charge density is difficult. For example, electron gun imperfections cause the electron beam to have a non-uniform space-charge density in a plane perpendicular to the Z-axis 28. Housing discontinuities 37 create gaps that prevent conventional ion clearing devices from subjecting the electron beam to an electric field in the upstream region. In compact systems, drift distance between the electron gun and the beam optics is relatively short, e.g., 40 cm or so, and the electron beam has insufficient time for its space-charge density to become sufficiently homogeneous.
Various specific ion controlling electrode assembly configurations were described in above-referenced U.S. patents to Rand et al. For example, FIG. 3 is a simplified depiction of ion controlling electrode assembly 44, based upon U.S. Pat. No. 5,289,519. This electrode assembly improves space-charge density and promotes sharp focusing of a high resolution X-ray image produced by system 8. Assembly 44 included a multi-sided rotatable field ion clearing electrode 46 ("RICE"), a washer-like positive ion electrode 48 ("PIE"), first and second multi-sided ion clearing electrodes 50, 50' ("ICE"), and a periodic axial field ion controlling electrode 52 ("PICE"), although not all of these elements were necessarily required. PICE 52 comprises a series of washer-like disks with alternate disks being connected to a common power source. The various RICE, PIE, ICE and PICE elements comprising assembly 44 preferably were stainless steel and were mounted within chamber 10 using insulated standoffs.
Electrode assembly 44 was mounted between electron gun 32 and beam optical assembly 38 within housing 10, with electron beam 12 passing axially through assembly 44 along Z-axis 28. Ideally Z-axis 28 is coaxial with electron beam 12 upstream from the beam optics assembly 38, and with both the longitudinal chamber axis and the axis of symmetry for electrode assembly 44 and beam optics assembly 38.
Typically elements comprising assembly 44 were coupled to one of several various sources of potential, e.g., Va, Vb, Vc, Vd, Ve, Vf, Vg. Typical values for these potentials were Va=0 V, Vb=-0.25 kV, Vc=-0.75 kV, Vd=-1 kV, Ve=-0.75 kV, Vf=-0.25 kV, and Vg=2 kV, In practice, the maximum potential Vd (which is -1 kV in the above example) could be between perhaps -0.8 kV and about -2 kV, with Vd, Vb, Vc, Ve, and Vf being scaled down or up proportionally.
These various potentials create transverse uniform electric fields to which the electron beam is subjected. Electric field non-uniformities, which may be represented by a multipole expansion, cause aberrations in the electron beam optics, to the detriment of the images produced by the overall system. The geometry of the three sets of electrodes and the applied potentials are selected to render zero the quadruple, sextupole, and octopole electric fields. As a result, the lowest order multipole electric field present is the decapole, which has minimal effects upon the electron beam.
Other electrode configurations are possible, for example those shown by electrode assemblies 46' and 46" in FIGS. 4A and 4B, respectively. Again, these assemblies produce a transverse uniform electric field. At any cross section, three sets of electrodes are present, energized at a nominal voltage Vd, Vd/2, and ground, where Vd is in the range of about -800 V to about -2 kV.
In general, as set forth in U.S. Pat. No. 5,239,519, dipole electric fields may be set to alternate in direction along the beam axis to minimize (but not completely eliminate) net deflection of the electron beam.
In viewing the configurations of FIGS. 3, 4A, and 4B, it is seen that multiple potentials are required. The requirement for multiple sources of operating potential, or (as shown in FIG. 4B) for divider circuitry to create multiple potentials is disadvantageous. It is inevitable with known ion clearing electrode systems that electric discharges will appear, with resultant inter-electrode voltage breakdown. When a discharge occurs the voltage on an electrode can change within microseconds to zero volts or to the voltage of an adjacent electrode, and can recover within milliseconds. Such electrode potential transients can affect the electron beam in several ways, none of which is beneficial to safe operation of the overall system.
Firstly, the potential change will deflect the electron beam because prior art electrode system are not truly non-deflecting in the presence of the beam-optical system solenoidal magnetic field. The deflected electron beam will not be on the axis of the beam-optical system after leaving the ion-clearing region. This can accentuate aberrations due to the final state of the ion-clearing device, or the downstream electron beam-optical components. Further, electrode discharge can interrupt uniform movement of the electron beam spot along the X-ray emitting target, perhaps momentarily slowing the traverse speed. The typically tungsten target surface can be seriously damaged by the decreased scan speed, and artifacts can appear in the reconstructed CT image. Even after the system recovers electrically, permanent damage to the tungsten target may have occurred, which will produce further artifacts.
Secondly, during a discharge the relative potentials of the various electrodes can change by different amounts, which will cause the electric field to become non-uniform. For example, one set of electrodes may short rapidly to ground potential while other electrodes might remain initially unaffected by the discharge. Further, the resistor-capacitor (RC) time constant associated with various sets of electrodes may differ, such that some electrodes will recover to their desired potential sooner than other electrodes. Resultant non-uniformities in the electric fields can generate quadrupole fields, which can unsafely focus the electron beam and cause damage to the tungsten target. Further, quadrupole and high order multipole fields can cause artifacts in the recovered CT system image.
In addition to such performance degradation, the requirement for separate power sources (FIGS. 3, 4A) or at least resistive voltage dividers (FIG. 4B) adds to the cost of the overall system. Further, reliability is compromised by the need to bring multiple electric feedthroughs into the vacuum chamber to couple the various potentials to ion clearing electrode system electrodes.
In summary, there is a need in electron beam scanner systems for a method and apparatus to remove positive ions, to prevent their neutralizing electron beam space-charge necessary to overcome upstream self-focusing. Such an ion clearing electrode method and apparatus should extract positive ions using electrostatic fields created from a single power source. Preferably the ion clearing electrode system should promote safe focusing regimes of the electron beam.